This invention relates to automatic pollution control systems for automotive vehicles, and more particularly to automatic devices adapted to reduce the amount of toxic pollutants in the exhaust by allowing controlled amounts of supplemental air, commonly known as "secondary air", to be mixed with the exhaust gases and to react therewith.
During normal operation of an internal combustion engine the reaction of the fuel and air produces a number of gases, particularly carbon-, hydrogen-, nitrogen-, and sulfur-based compounds. Considerable heating of the exhaust manifold occurs as a result of conduction of the heat contained in the exhaust gas to the metal surfaces thereof. It has been found that some of the exhaust gases from an internal combustion engine can be burned further by the introduction of controlled quantities of air under certain conditions, thereby either reducing the net pollutants emanating from the engine, or else rendering such pollutants less toxic, as for example, converting poisonous carbon monoxide to harmless carbon dioxide and water vapor.
The additional heat generated by the burning process results in increased temperatures in the manifolds. While this has been found to be not objectionable at low and mid-range engine speeds, excessive heat tends to occur if the engine speed increases above a certain point. Empirically it has been determined that a safe temperature can be maintained at engine speeds of around 3000 r.p.m. or less; above this figure it is necessary to divert the secondary air away from the manifolds, and either route it back into the air strainer or directly to the muffler. While this diverting inhibits the desired "burning" mentioned above, it prevents undesirable overheating of and damage to the manifolds. If allowed to continue at high engine speeds, the "burning" process could cause permanent damage to the exhaust system.
It has been proposed that one or more solenoid-operated air-diverter valves be employed, to provide communication with the source of the secondary air and the manifolds, such valves being adapted to be opened by the solenoids when the vehicle is initially started, and remain open when the engine is run at speeds below a predetermined or given speed. In the event that speed is increased, the solenoids following a suitable delay could be de-energized, thus closing the valves and eliminating the overheating.